The present invention relates to nuclear fuel-containing plate structures for use in nuclear fission reactors. In particular, the present invention relates to high loading uranium fuel plates and to their method of fabrication.
In certain known types of nuclear fission reactors, particularly research reactors, it is known to employ nuclear fuel-containing plate structures to provide the fissionable nuclear fuel, such as uranium or uranium compounds, enriched with uranium 235. In such known plate structures, a fissionable fuel material or "meat" is sandwiched between aluminum cladding plates which afford structural support and containment for the fission products.
In some cases, a uranium aluminum alloy has been used for the meat. This material has excellent behavior under neutron irradiation, in that the material does not swell excessively or otherwise cause trouble, but this material is limited to a low uranium density of approximately 1.3 grams of uranium per cubic centimeter. Because of the low uranium density, it has generally been necessary to use uranium having a high percentage of enrichment with uranium 235. The high enrichment percentage has a disadvantage in that the uranium can readily be reprocessed for use in nuclear weapons.
More recently, dispersion fuels have been used to obtain higher uranium densities in the meat. To produce such dispersion fuels, fine powders of pure aluminum and uranium compounds have been mixed (dispersed) together and pressed into a polyhedron having six substantially rectangular faces to form a dispersion compact for use as the meat. The uranium densities this achieved depend upon the choice of the uranium compound and on its volumetric fraction in the fuel meat. The largest uranium density achieved in commercially produced dispersion compacts has been about 4.8 grams of uranium per cubic centimeter (gU/cm.sup.3).
The loading limits of current conventional fabrication techniques have been reached for U.sub.3 Si and U.sub.3 Si.sub.2 powder metallurgy fuel plates. On a laboratory scale, fuel loadings of up to 7.0 gU/cm.sup.3 have been successfully produced. There are a smaller number of reactors, however, that require higher fuel loadings. As a point of reference for reactors loaded with HEU enriched UAl.sub.x to 1.7 gU/cm.sup.3 (high enrichment uranium at 93% uranium 235) and a conventional fuel meat thickness of 0.020 inches (0.51 mm), the equivalent loading for LEU (low enrichment uranium at 20% uranium 234 would be: ##EQU1##
From a quality and a fabrication point of view, this level of uranium loading is well beyond the loading range possible for conventionally produced powder metallurgy core fuel plates having a meat thickness of 0.020 inch, regardless of the fuel alloy chosen.
The most common problem with highly loaded fuel plates is the limitation of the material system to uniformly respond to the conventional rolling process. The major cause of this is the difference in the hardness of the fuel in the matrix. (The matrix is defined as the aluminum powder intermingled with the uranium powder plus the aluminum alloy of the cladding so that the matrix, in effect, is all metal which surrounds the uranium compound.) The fuel particles are very hard and resist deformation, whereas the matrix is comparatively soft and flows very easily. As the volume fraction of the harder fuel phase in the meat is increased, the classic "dog-bone" effect is seen at about 40 to 50 volume percent. This is a thickening of the ends of the fuel zone such that when viewed in longitudinal cross-section, the fuel zone profile resembles a dog-bone.
The ends of the fuel zone are where the most non-equilibrium conditions occur. In this region, the aluminum cladding is forced over the fuel zone without reducing the meat thickness and a dog-bone occurs. In the center of the fuel zone, the constraint on the meat is about equal during the rolling process, and the deformation is uniform. At the end of the fuel zone the same conditions exist as at the beginning, and this end also thickens. Shaped (tapered) dispersion compacts can reduce but not eliminate this effect up to about 55 volume percent fuel. At higher fuel percentages, the differences in hardness are so great that a fuel zone is generated throughout the whole meat mass which is a series of thick and thin areas, thus providing a series of dog-bones. Because of minimum cladding thickness requirements, as well as uranium uniformity limits, plates with this shape of fuel zone are not acceptable.
The root of this problem is in the conventional rolling process. It is a line contact deformation system. The meat is constrained from lateral motion by the friction between the cladding surface and the line of contact with the rollers. However, the meat can move longitudinally in the rolling direction, since this is the least restrained direction, and dog-boning occurs. This flow is probably greatest during the early rolling passes when the meat is in the least constrained by thickness and geometry. At later passes, there is more resistance and probably less dog-boning.
Because of the dog-boning problem, and the accompanying problems of non-uniform and insufficient cladding thickness, non-uniform fuel loading, and fuel out-of-zone, the conventional rolling process places constraints upon the structure of the prior art uranium fuel plates. For example, for a conventional fuel plate having a thickness of 0.050 inch, the cladding above and below the meat must be 0.015 inch in order to allow for thickness irregularities caused by the conventional rolling process. This means that the meat must have a thickness not greater than 0.020 inch. It is this thickness limit which has caused the present uranium loading limit. Thus, conventional fuel plates having a thickness of 0.050 inch contain a dispersioin compact (meat) having a thickness not greater than 0.020 inch wherein the uranium fuel volume is not greater than about 45 Vol.%, the porosity is about 10 Vol.%, and the aluminum volume in the meat is the balance.
With this then being the state of the art, it an object of the present invention to provide new and improved nuclear fuel-containing plate structures having increased uranium loading.
It is another object of the present invention to provide new and improved nuclear fuel-containing plate structures having increased meat density and reduced porosity.
It is a further object of the present invention to provide such plate structures which have improved structural integrity and uniformity with effective containment of the nuclear fission products.
These and other objects of the present invention, as well as the advantages thereof, will become more clear from the description which follows.